A number of submersible pumps used to pump molten metal (referred to herein as molten metal pumps) are known in the art. For example, U.S. Pat. No. 2,948,524 to Sweeney et al., U.S. Pat. No. 4,169,584 to Mangalick, U.S. Pat. No. 5,203,681 to Cooper, and pending U.S. patent application Ser. No. 08/439,739 to Cooper, the disclosures of which are incorporated herein by reference, all disclose molten metal pumps. The term submersible means that when the pump is in use, its base is submerged in a bath of molten metal.
Three basic types of pumps for pumping molten metal, such as aluminum, are utilized, circulation pumps, transfer pumps and gas-release pumps. Circulation pumps are used to circulate the molten metal within a bath, thereby equalizing the temperature of the molten metal and creating a uniformly consistent alloy. Most often, as is known by those skilled in the art, circulation pumps are used in conjunction with a reverbatory furnace having an external well. The well is usually an extension of the charging well where scrap metal is charged (i.e., added).
Transfer pumps are generally used to transfer molten metal from the external well of the furnace to a different location such as a ladle or another furnace.
Gas-release pumps, such as gas-injection pumps, circulate the molten metal while adding a gas into the flow of molten metal in order to "demag" or "degas" the molten metal. In the purification of molten metals, particularly aluminum, it is frequently desired to remove dissolved gases such as hydrogen, or dissolved metals, such as magnesium. As is known by those skilled in the art, the removing of dissolved gas is known as "degassing" while the removal of magnesium is known as "demagging."
All molten-metal pumps include a pump base that comprises a housing, also called a casing, a pump chamber, which is an open area formed within the housing, and a discharge, which is a channel or conduit communicating with the chamber and leading from the chamber to an outlet formed in the exterior of the casing. A rotor, also called an impeller, is mounted in the pump chamber and connected to a drive system, which is typically one or more vertical shafts that eventually connect to a motor. As the drive system turns the rotor, the rotor pushes molten metal out of the pump chamber, through the discharge, out of the outlet and into the molten metal bath.
A bearing member is added to the pump casing, which is preferably a ceramic ring attached to the bottom edge of the chamber. The inner perimeter of the ring forms a first bearing surface. A corresponding bearing member, which is a ceramic ring (sometimes referred to as a rotor ring), is attached to the rotor, and its outer perimeter forms a second bearing surface. The rotor is vertically aligned in the pump chamber so that the second bearing surface of the rotor aligns with the first bearing surface of the pump chamber. When the rotor turns, the first bearing surface keeps the second bearing surface centered, which in turn keeps the rotor centered in the pump chamber.
A problem encountered with this arrangement is that the ceramic ring attached to the rotor is fragile and often breaks. It breaks during operation of the pump because of impact against the bearing surface or because pieces of solid material, such as brick or dross present within the aluminum bath, become wedged between the bearing surface and the second bearing surface. The ceramic ring attached to the rotor also breaks during start up because of thermal expansion. In this respect, whenever a rotor including a rotor ring is placed in the pump, the ring is quickly heated from the ambient air temperature within the factory to the temperature of molten aluminum. The ring then expands and can crack. To alleviate cracking due to thermal expansion, the furnace operator may slowly heat the entire furnace to prevent thermal shock to the ring, but this results in downtime and lost production. Finally, the rings are easily damaged during shipping.